This invention relates to printed circuit boards and methods of fabricating such boards. In particular, the invention relates to a printed circuit board on which electronic parts can be attached without using soldering techniques or anisotropic electrically conductive adhesive techniques. The invention also relates to methods of attaching electronic parts, or other circuit boards, to a circuit board.
In the past, circuit patterns on circuit board substrates were mainly made from copper and in order to connect electronic parts to copper patterns, the parts were soldered directly onto the pattern. The patterns were usually prepared by etching and removing copper foil which was laminated onto the substrate. More recently, and alternatively, the circuit pattern can be formed by printing a conductive paste containing electrically conductive particles such as silver, platinum or palladium and glass frit, followed by sintering.
However, both methods entail problems. The former method of preparation involves the complicated process of etching. It also incurs waste of material because a major part of the copper foil is removed and discarded. The latter method requires baking at high temperatures in the range of 600.degree.-900.degree. C., and requires the use of expensive and easily damaged ceramics as the substrate. In addition, the latter method requires baking equipment which is large and highly bulky.
As a result, attempts have been made to acquire an electrically conductive composition which leads to a simpler process for making solderable circuits and which uses inexpensive material. One attempt involves using conductive paste compositions which employ a synthetic resin binder that can be cured at a relatively low temperature. Such electrically conductive paste is formed, for example, by dispersing silver powder in epoxy resin or by dispersing copper powder in phenolic resin. The paste is then printed onto a synthetic resin laminated board or film and baked to form the cured circuit pattern.
Another attempt involves a method which does not require a soldering process to connect electronic parts or other circuit boards to the circuitry. Such a method involves using anisotropic electrically conductive adhesives. See U.S. Pat. No. 4,133,981 to Fujita et al. Those adhesives are prepared by dispersing electrically conductive metallic particles or powder in a binder resin composition. The binder resin and dispersed particles are then prepared in film form and adhered on one of the two sets of terminals to be connected. The two sets of terminals are then adhered together by heating the terminals with a heat press. However, this method, as well as the conductive paste method, have the following problems.
When using electrically conductive paste compositions, the bonding strength between the resin paste and the solder can be low. When using high amounts of resin binder the likelihood of a sufficient amount of conductive particles being exposed is reduced. Thus, contact with the solder is reduced. Further, while it is possible to increase the bonding strength with the solder by increasing the amount of the electrically conductive particles, such practice uses a smaller amount of resin and thus creates a new problem of reducing the adhesion strength between the insulating substrate and the conductive circuits. In addition, solders which have extremely low melting points possess poor solderability and are useless when using such methods because there is little bonding strength between the conductive paste and such solders. When silver powder is used as the electrically conductive particles in such pastes, a problem called "silver biting" occurs when the silver powder diffuses into the solder. This problem leads to reduced or lost conductivity between the part being attached by the solder and the circuit traces or pads. When copper powder is used as the electrically conductive particles, the surface of the copper powder may oxidize and a stable solder is thus difficult to obtain.
As mentioned above, methods using anisotropic electrically conductive adhesives also incur difficulties. For instance, it is difficult to avoid aggregation of the electrically conductive particles when fabricating the adhesives. It is also extremely difficult to prevent re-aggregation of the particles during the bonding process used to connect the opposing sets of terminals. As a result of this aggregation problem, particularly when the circuit is extremely fine, it is difficult to totally eliminate short circuiting with the neighboring terminals. Therefore a thorough inspection for and repair of any defects in the products is sometimes necessary after the connection step. Further, the anisotropic adhesive requires additional steps. For instance, if there are many parts which require bonding onto the circuit board, it is necessary to transfer the anisotropic electrically conductive adhesive from a carrier film to each area to which a part will be attached. It is also necessary that the adhesive film be shaped to match the shape of the area to which the part will be attached. This step not only requires more time and work, but is difficult to automate. Even moreover, the anisotropic electrically conductive adhesives tend to be opaque due to the presence of the conductive particles. That opacity causes a difficult problem with the precise alignment of the positions in the heat press/bonding process of the parts, particularly when the circuit is extremely fine in pitch.